It is known in the art to supply forced air at some temperature above or below the temperature within a room in order to control the temperature within a room by raising or lowering the temperature as desired. Such systems include some means for supplying temperature regulated air, including air-conditioning units and/or heating units which supply air into a duct system within a building. Each zone within the building in which temperature is desired to be regulated is fed air from the duct system and includes a ventilation opening regulated by a damper. The air flow through the damper is regulated by a damper valve which is controlled by a damper control system. In some systems damper valve position is controlled by a closed loop in which actual damper valve position, sensed by a potentiometer, for example, is used as a feedback signal for comparison with a temperature error signal.
In the past it has been typical to regulate air flow through the damper by controlling the damper valve responsive to the temperature within the zone being regulated by the particular damper. A thermostat within a room supplied by the damper thus is typically used to provide a temperature error signal to the damper control valve, which is representative of the difference between the desired temperature in the room and the measured temperature as measured by the thermostat within the room. Damper valve position is thus controlled by the temperature signal.
Such systems suffer from a number of drawbacks, due largely to the fact that the pressure and the temperature of the air supplied to each individual ventilation opening for each individual zone throughout the system vary according to the position of the damper valve within the ducting system with respect to the source of the temperature controlled air. Thus, the dampers furthest away from the heating or cooling units will receive air which is at a lower pressure due to the loss of pressure resulting from supplying air to ventilation openings closer to the source of heated or cooled air. In addition, the temperature varies somewhat due to the losses in the surrounding environment, even in a relatively well insulated ducting sytsem.
Thus, in a system according to the prior art where the damper valve position is set according to the difference between the temperature in the room and the desired temperature in the room, fluctuations in the pressure at the particular damper valve, and also fluctuations in the temperature of the air supplied at the damper valve can often result in an error in the amount of air supplied to the room necessary to bring the room to the set temperature. Errors in desired temperature also occur when temperatures within other rooms throughout the system are less than or greater than the selected set temperature for the room, resulting in changes in demand at each particular damper valve. Such changes modify the pressure available at other damper valves. Moreover, the damper valves themselves typically are not linear throughout their range of openings, or if they are designed to be approximately linear throughout their range of openings, they can be expensive to manufacture. The result of any nonlinearity throughout the range of openings of the damper valve is that in a temperature feedback system the change in the air flow resulting from a signal indicating that the damper should be positioned to affect a four degree temperature change will likely not be twice the air flow resulting from a signal indicating the damper valve should be positioned to effect a two degree temperature change.
These drawbacks can cause the system to become out of balance and remain out of balance for long periods of time and to needlessly hunt within a particular temperature control zone to finally attain the set temperature within the zone as measured by the thermostat within the zone.
A multizone temperature control system for a building furnishes a supply of air, heated or cooled to a preselected temperature, to all or a selected group of the temperature control zones by means of a central fan or fans that blow air over a number, such as two, three or more, of heat exchangers, each coupled with a cooling compressor or a heater. The fan provides a predetermined quantity of air to the heat exchangers. Part of the temperature controlled air from the heat exchangers is fed to the individual temperature control zones, and another part is bypassed back to the fan input. It is common, at present, to operate the temperature control units, (e.g. the compressors or the heaters) according to temperature sensed in individual temperature control zones. A single one of the temperature control units (compressor or heater) is operated when sensed temperature is at or close to the desired point. Second, third or other temperature control units are turned on as sensed temperature departs further from the set point. For example, according to prior practice, if the temperature sensor in one temperature control zone of highest temperature is one degree or less above its set point, only one cooler compressor is operated. Should the temperature in such highest temperature control zone rise to more than two degrees above the set point at such temperature control zone, the second compressor is turned on and the two operate together. The second compressor is turned off if sensed temperature drops below a selected level. Therefore, the compressors may be turned on and off rapidly, which is damaging to the compressors. To alleviate this problem, prior systems may employ timers to prevent short period compressor cycling. Such timers inherently operate to delay a desired turn on or turn off of a compressor, and thus introduce further control error. In a system in which the control for selection of the number of operating compressors or heaters is based upon a temperature sensor in only the one highest temperature control zone, it is quite possible that the system may call for a second compressor to operate when temperature of only one of a large number of temperature control zones is too high. In other words, although only a small fraction of the cooling capacity of the system is needed to handle the raised temperature of the one temperature control zone, the system may call for operation of several compressors. In such a situation, either the air flowing to all of the temperature control zones will be cooled to an undesirably low temperature, or the compressors may freeze.
Many prior systems do not use a bypass around the system fan to divert air not needed for temperature control of the various zones. These arrangements required complex and inefficient methods to balance the air actually used and the fan output capacity, often requiring different arrangements for controlling the fan. In one prior system, the total amount of temperature controlled air fed to all of the temperature control zones collectively is varied in accordance wtih velocity of air flow in the air conditioner unit itself. If velocity slows down (indicating a build up of pressure), a fan bypass is opened to allow the velocity through the air-conditioner to increase. With such a system it is difficult to set up the air balance. To set up air balance, initially all zone dampers must be opened and the bypass is set so it is just ready to close. Then as zone dampers begin to close in response to temperature rise, pressure increases, velocity in the air-conditioner itself decreases, and the bypass will begin to open. Accurate balance of the air flow is very difficult in such a system. Such a system, in effect, senses increased pressure which is manifested as a decrease in velocity through the air-conditioner unit, but such pressure changes inherently and undesirably affect air flow to all zones.
Recognizing the need for an improved method and apparatus for regulating the air flow within temperature control zones within a building, and for regulating the temperature controlling air supply, it is a principal object of the present invention to provide an improved apparatus and method for such regulation.
A system embodying principles of the present invention employs air flow velocity sensors within the particular zone dampers. A control signal from the temperature sensor within the zone of temperature control is arranged to indicate a target velocity for the air flow into the zone of control. Control of zone temperature is obtained by modifying the damper valve position until the target velocity is obtained. In one sense damper position is controlled by a feedback loop in which sensed velocity is used to indicate damper valve position. The velocity sensed in the damper forms a feedback signal to the means for moving the damper valve to a position to attain the target velocity. The system also includes an improved apparatus for sensing the air flow velocity in the damper which is generally representative of the average air flow velocity throughout the cross section of the damper.
According to other features of the invention, both the fan bypass damper of the air supply system and the number of temperature control units in use are controlled in accordance with the total demand air flow, namely the total air actually being used by all of the temperature control zones collectively. Individual zone velocity sensors are used to provide a signal representing the total quantity of air used by all zones, which signal is compared with a signal representing total capacity of the fan, so as to control the fan bypass and to divert from the temperature control zones the difference between fan output capacity and the amount of air that is actually being used. A velocity based loop is also employed to control air flow through the fan bypass, directly sensing such velocity, for control of the bypass damper valve position.
For control of the number of temperature control units (heater or air conditioners), the output capacity of the fan is divided into a plurality of air flow quantity ranges so that the number of temperature control units activated at any given time depends upon the relation of the total demand air flow, as measured by all of the zone velocity sensors collectively, to the several air flow quantity ranges. For example, when total demand air flow is within a lower range, only one temperature control unit is activated. When total demand air flow is within a second higher range, a second temperature control unit is activated so that the two are operating together. According to one aspect of this operation, the adjacent ranges overlap to a small extent to decrease excessive on/off recycling.